1. Field of the Invention
The present invention relates to an improved nickel positive electrode for an alkaline storage battery, and also to a sealed type nickel-hydrogen storage battery using the nickel positive electrode.
2. Description of the Related Art
Recent trend for high value-added and down-sized portable apparatus has significantly enhanced the requirement for high energy density secondary cells. Development of new secondary cells having a high energy density is also highly required as a battery of electric vehicles. Alkaline storage batteries recently developed to meet these demands include a high-capacity nickel-cadmium storage battery with a conventional sintered nickel positive electrode, and a high energy-density nickel-cadmium storage battery with a foamed nickel positive electrode having a capacity 30 to 60% higher than that of the conventional electrode. Another example is a nickel-hydrogen storage battery using a hydrogen storage alloy as a negative electrode, which has a higher capacity than the nickel-cadmium storage batteries.
In such high-capacity alkaline storage batteries, nickel hydroxide powder is closely packed into a sintered nickel porous substrate, a three-dimensional foamed nickel porous substrate having the high porosity of not less than 90% or a nickel fiber porous substrate in order to improve the energy density of the positive electrode. The closely packed nickel hydroxide has improved the energy density to 450 to 500 mAh/cm.sup.3 for the recent sintered nickel positive electrodes and to 550 to 650 mAh/cm.sup.3 for the foamed nickel positive electrodes, compared to the energy density of 400 to 450 mAh/cm.sup.3 for the conventional sintered nickel positive electrodes.
In such positive electrodes prepared by closely packing nickel hydroxide powder in a sintered nickel porous substrate, a foamed nickel porous substrate or a nickel fiber porous substrate, the packing density is increased by application of pressure. Application of the pressure, however, causes expansion of electrode plates and compresses separators each placed between a positive electrode and a negative electrode in the course of repetitive charging and discharging. The compression of the separators presses out an electrolyte solution included in the separators which makes a significant contribution to charge and discharge characteristics, thus deteriorating the discharge characteristics. A battery is sometimes discharged to approximately 0 V by an accidental continuous power-on of a portable apparatus. Repeated charging and discharging of such a battery significantly lowers the discharge voltage and increases the internal impedance.
The positive electrode prepared by closely packing nickel hydroxide powder in a sintered nickel porous substrate, a foamed nickel porous substrate or a nickel fiber porous substrate has a high energy density at ordinary temperatures but has a relatively low energy density in a high-temperature atmosphere. Namely, the merit of the high energy density can not be exerted in a wide temperature range. Charging under a high-temperature atmosphere generates oxygen simultaneously with a discharge reaction of converting nickel hydroxide to nickel oxyhydroxide. In other words, this decreases the overvoltage for evolution of oxygen at the positive electrode and prevents the discharge reaction of converting nickel hydroxide to nickel oxyhydroxide, thus lowering the utilization of nickel hydroxide.
Several methods have been proposed to solve the above-mentioned problems:
(1) A method of adding cadmium oxide powder or cadmium hydroxide powder to a positive electrode; PA0 (2) A method of making a cadmium oxide contained in nickel hydroxide powder (disclosed in Japanese Laid-Open Patent No. 61-104565); and PA0 (3) A method of adding a powdery compound of a IIa group element, such as, calcium hydroxide to a sintered nickel positive electrode (disclosed in Japanese Laid-Open Patent No. 48-46841 and U.S. Pat. No. 3,826,684).
An active material mixture supported by a conductive support such as a sintered nickel porous substrate, a foamed nickel porous substrate or a nickel fiber porous substrate includes metal cobalt powder for enhancing the utilization of the active material and metal nickel powder as a conductive agent. Cobalt hydroxide or cobalt oxide may be used in place of the metal cobalt powder. In the positive electrode including such an actlye materlal mixture, repeated charging and discharging cycles having a large discharge depth, for example, discharge to approximately 0 V, lower the discharge voltage and increase the internal impedance to shorten the cycle life. This problem can not be solved by simply adjusting the quantity of metal cobalt powder or metal nickel powder added to the positive electrode.
In the method (1) or (2), a cadmium oxide is included in or mixed with nickel hydroxide powder to improve the utilization of nickel hydroxide in a high-temperature atmosphere. Addition of the cadmium oxide, however, improves the utilization of nickel hydroxide only to approximately 80% in the high-temperature atmosphere. A larger quantity of the cadmium oxide included in or mixed with nickel hydroxide powder is essential for further improvement of the utilization of nickel hydroxide in the high-temperature atmosphere. While actually improving the utilization of nickel hydroxide to approximately 90% in the high-temperature atmosphere, the large quantity of the cadmium oxide lowers the utilization of nickel hydroxide at ordinary temperatures. Addition of heavy metal, cadmium compounds is not favorable for protection of the environment.
In the method (3), a sintered nickel positive electrode is immersed first in an aqueous solution of calcium nitrate and then in an aqueous solution of sodium hydroxide. Calcium hydroxide thus precipitated is added to the positive electrode to improve the utilization of nickel hydroxide in a high-temperature atmosphere. Like the above methods (1) and (2), addition of calcium hydroxide improves the utilization of nickel hydroxide in the high-temperature atmosphere while lowering the utilization of nickel hydroxide at ordinary temperatures. In the method (3), calcium hydroxide is added to the sintered nickel positive electrode by immersing the electrode in a calcium nitrate solution. This causes the residual nitrates to exist in the nickel positive electrode. In a sealed battery with such a sintered nickel positive electrode, the residual nitrates undesirably increase the self discharge.
Addition of calcium hydroxide powder to the paste-type nickel positive electrode does not improve the utilization at ordinary and high temperatures.